1. Field of the Invention
The present invention relates to methods and apparatuses for detecting defects in an object having projected portions formed in the same shape with a predetermined pitch along an arc, the object including, for example, an external gear, an internal gear, a sprocket, or a mechanical element having projected portions equivalent to those modules along part of an arc. More particularly, the present invention relates to a method and apparatus preferably used for detecting a chipped portion in an object such as castings or sintered products, on the tip of the projected portion of which a defect or flaw may be easily formed.
2. Description of the Related Art
Known as a defect detection method and apparatus of this type is such that the reference image data of a good object is predetermined and then the image data of an inspected object is compared with the reference data to thereby determine if a rejectable defect exists on the object.
Now, a conventional defect detection method and apparatus of this type will be briefly explained below with reference to the conceptual view of a processing procedure shown in FIG. 8 and the view of the processing principle shown in FIG. 9.
In the conventional defect detection method and apparatus, first, as the pre-processing for the actual inspection of a defect, an object having a proper shape with no defect is positioned in place and then the image data of the object is obtained as shown in FIG. 9(a) (step a1 of FIG. 8).
Subsequently, the image data is stored in an image processing apparatus or the like as the reference image data for determining if a defect exists on an inspected object (step a2 of FIG. 8). As shown in FIG. 9(a), the frame of image data contains information on the position of the object, that is, information as to where the object is placed in the view field of the camera. More specifically, this positional information represents addresses in a frame memory.
In addition, in this pre-processing step, an operator manually inputs criteria such as allowable errors in dimension or shape of the object for determining that the object is acceptable, considering the properties of the object such as the shape, size or the like (step a3 of FIG. 8). Excessively exacting tolerances would provide excessively tightened GO/NO-GO criteria, whereas loosely determined tolerances would provide inaccurate GO/NO-GO criteria.
Upon inspection of a newly manufactured object, the object to be inspected is placed in the same position as that of the good object placed to prepare the reference image data mentioned above (step a4 of FIG. 8). Then, a frame of image data of the inspected object is obtained in the same manner as described above as shown in FIG. 9(b) (step a5 of FIG. 8). Incidentally, as shown in FIG. 9(b), it should be understood that the inspected object is supposed to have a defect on its circumference portion in its image frame, with a portion corresponding to the defect being indicated qualitatively with a hollow circle.
Then, the image data registered beforehand as described above, or the reference for a determination of whether the object is acceptable, is read into the image processing apparatus, and thereafter the reference image data and the currently captured image data of the inspected object are compared with each other to output the difference therebetween (step a6 of FIG. 8). In practice, the two frames of image data are compared bit by bit with each other in the frame memory of the image processing apparatus to output the portions having inconsistencies in density. Accordingly, the difference between the reference data of FIG. 9(a) and the inspection data of FIG. 9(b) can be outputted qualitatively in the form of a hollow circle as shown in FIG. 9(c).
In other words, the size of the area of the circle indicates the degree of inconsistency in density between the two, with a smaller area indicating a less degree of inconsistency and a larger area indicating a higher degree of inconsistency.
Thus, it is finally determined that the inspected object is acceptable if the degree of inconsistency or the output level of difference does not exceed the aforementioned criteria but determined that the object is rejectable if the output level of difference exceeds the criteria (step a7 of FIG. 8).
However, as described above, the frames of image data of FIGS. 9(a) and 9(b) contain information regarding the position of the object in s frame. If the object is not properly positioned in place upon preparation of reference image data or inspection of the object, this would provide an increased output level of difference due to a shift in position and thereby result in determining that the object is rejectable even if the object is actually acceptable.
On the other hand, if the object has a generally round outer diametric contour such as an external gear, an internal gear, or a sprocket and has been positioned in place exactly in the same way, even when the outer periphery of the object abuts a jig or the like and the object is thereby positioned precisely in place, a shift in orientation caused by the rotation of the object in the place cannot be controlled. Thus, the shift in rotational orientation of the object would lead to an inconsistency in position of the projected portions on the outer periphery even when the outer diametric portion of the object is pushed against the jig or the like to position the center of the object with accuracy. This inconsistency would cause the aforementioned output level of difference to increase, thereby making it rather difficult to properly determine whether a defect exists on the object.
As described above, the conventional defect detection method and apparatus of this type had a drawback of making it extremely difficult to determine whether a defect exists on the object, due to a shift in position of the placement or in rotational orientation of the object.
In addition, as is obvious from the aforementioned operation principle, it is necessary to prepare individual reference data and criteria for each size and shape of an inspected object of a different type in order to determine whether the object is acceptable. Thus, this raises such problems that the preparation is laborious and requires storage means having an increased capacity for storing a plurality of types of reference data and criteria.
The GO/NO-GO criteria are also affected by a shift in position of the placement and in rotational orientation of the object, and thus no reasonable guidance is available for determining the magnitude of the criteria. This raises a problem of making it difficult to provide criteria which allow a precise determination of whether the object is acceptable.
Furthermore, with the conventional apparatus of this type, it is commonly practiced that an inspected object is illuminated with light and then the reflected light is used for imaging the object to generate image data thereof. Thus, in some cases, depending on the relationship between the color of the inspected object and a background color, the apparatus would have a drawback of making it impossible to provide a sufficient contrast required for generating a binary image from the image information of the object.
Furthermore, shades or the like caused by ambient light have an effect on a determination of whether the object is acceptable. Thus, it is very difficult to always provide an optimum inspection environment.
It is therefore an object of the present invention to provide a defect detection method and system which eliminate the aforementioned drawbacks of the prior art, which always provides a correct determination of whether an object such as a gear or a sprocket is acceptable even with a shift in position of placement or in rotational orientation of the object, and which require no laborious preparation and facilitate the setting of GO/NO-GO criteria.
The present invention provides a method for detecting a defect on an object having projected portions formed in the same shape along an arc with a predetermined pitch. In particular, the present invention provides a method for detecting a defect on the projected portions of an object having the projected portions outside an arc such as an external gear or a sprocket. The method includes the steps of determining an arc circumscribing a tip of each projected portion of an inspected object, and extracting each overlapping region formed by an overlapping portion between an inner portion of a region defined by the arc and a cut-away portion of the object to determine an area of each overlapping region. The method further includes the steps of comparing an area of each of the overlapping regions with each other, and determining that no defect exists on the object if each area difference falls within a range of predetermined criteria, whereas determining that a defect exists on the object if the area difference exceeds the range of the predetermined criteria.
The size of each overlapping region formed by the inner portion of an arc circumscribing the tip of each projected portion of the inspected object and a cut-away portion of the object contains no information regarding the position of the object and is always constant irrespective of the position and rotational orientation of the object. Therefore, a determination of whether the object is acceptable by comparing the area of these overlapping regions with each other makes it possible to provide a precise determination whether the object is acceptable, irrespective of a shift in position or in rotational orientation of the inspected object.
If an anomaly such as a chipped portion has occurred on the tip of a projected portion, adjacent overlapping regions of the aforementioned overlapping regions are not separated from but integrated with each other, thereby considerably increasing the area of an overlapping region adjacent the chipped portion. More specifically, the area of the overlapping region is two times or more the area of a normal overlapping region. As such, the occurrence of an anomaly such as a chipped portion causes the area of an overlapping region to change significantly in a discontinuous manner. This makes it possible to set easily the criteria to be used for a determination of whether the object is acceptable as well as to detect positively a chipped portion or the like occurring on the tip of a projected portion.
Furthermore, the data to be used for a determination of whether the object is acceptable is extracted from the inspected object itself upon inspection of the object. This obviates the need of the pre-processing for generating reference data in advance using a good sample, thereby simplifying the preparation. In addition, the data to be used for a determination of whether the object is acceptable is extracted from the inspected object itself. This prevents the size and shape of the object from affecting the result of a determination of whether the object is acceptable. Moreover, this obviates the need of setting reference data and criteria by the type of inspected objects, thereby making it unnecessary to use a high-capacity storage means for storing these data.
The present invention provides a system for detecting a defect on an object having projected portions formed in the same shape along an arc with a predetermined pitch. In particular, the present invention provides a system for detecting a defect on projected portions of an object having the projected portions outside an arc such as an external gear or a sprocket. The system includes imaging means for imaging the inspected object, and image capture means for holding an picked-up image as digital data. The system also includes region area detection means for analyzing the digital data held by the image capture means to determine an arc circumscribing a tip of each of the projected portion of the object, and then extracting each overlapping region formed by an overlapping portion between an inner portion of a region defined by the arc and a cut-away portion of the object to determine an area of each overlapping region. The system further includes region area comparison means for comparing an area of each overlapping region determined by the region area detection means with each other to determine an area difference, and defect determination means for determining that no defect exists on the object if the area difference determined by the region area comparison means falls within a range of predetermined criteria, whereas determining that a defect exists on the object if the area difference exceeds the range of the predetermined criteria.
With this configuration, the inspected object is imaged with the imaging means and then the digital data of the resulting image is held by means of the image capture means.
Then, the region area detection means analyzes the digital data held by the image capture means to determine an arc circumscribing the tip of each of the projected portion of the object, and then extracts each overlapping region formed by an overlapping portion between an inner portion of a region defined by the arc and a cut-away portion of the object to determine an area of each overlapping region.
The region area comparison means compares an area of each overlapping region determined by the region area detection means with each other to determine an area difference. Finally, the defect determination means outputs the result of a determination indicative of the absence of defects if the area difference determined by the region area comparison means falls within a range of predetermined criteria, whereas outputting the result of a determination indicative of the presence of a defect if the area difference exceeds the range of the predetermined criteria.
The present invention also provides a method for detecting a defect on an object having projected portions inside an arc such as an internal gear, the method including the following steps to achieve the aforementioned object. That is, the method includes the steps of determining an arc inscribing a tip of each projected portion of an inspected object, and extracting each overlapping region formed by an overlapping portion between an outer portion of a region defined by the arc and a cut-away portion of the object to determine an area of each overlapping region. The method also includes the steps of comparing an area of each of the overlapping regions with each other, and determining that no defect exists on the object if each area difference falls within a range of predetermined criteria, whereas determining that a defect exists on the object if the area difference exceeds the range of the predetermined criteria.
As such, the method for detecting a defect on an object having projected portions inside an arc includes the step of comparing overlapping regions with each other, each overlapping region being formed by an overlapping portion between an outer portion of a region defined by the arc inscribing the tip of each projected portion and a cut-away portion of the object. Other points are the same as those of the method for detecting a defect on an object having projected portions outside an arc.
In addition, the present invention also provides a system for detecting a defect on an object having projected portions inside an arc such as an internal gear. This system allows region area detection means to extract each overlapping region formed by an overlapping portion between an outer portion of a region defined by the arc inscribing the projected portions and a cut-away portion of the object to determine an area of each overlapping region and compare the values with each other.
Other points are the same as those of the system for detecting a defect on an object having projected portions outside an arc.
Furthermore, these defect detection systems further include a lighting box for placing the inspected object thereon, and the imaging means is arranged opposite to the illuminating surface of the lighting box, thereby providing a sufficient contrast required for image processing.
In this case, the contour of the inspected object is detected through a determination of whether the transmitted light from the lighting box is blocked. This makes it possible to precisely image the contour of the inspected object irrespective of the color of the object or the effect of ambient light.
In addition, the imaging means of these defect detection systems can be provided with a band pass filter for eliminating, as deleterious light, light having wavelengths other than those of light for the lighting box to illuminate the object with.
This ensures the elimination of the effect exerted by the intensity of ambient light and the effect of shades produced thereby.
The nature, principle, and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.